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                    <h1 class="description center-align post-title">PriorityQueue的用法和底层实现原理</h1>
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                <h1 id="先讲使用，再讲原理"><a href="#先讲使用，再讲原理" class="headerlink" title="先讲使用，再讲原理"></a>先讲使用，再讲原理</h1><p>举两个例子：</p>
<p>作业系统中的调度程序，当一个作业完成后，需要在所有等待调度的作业中选择一个优先级最高的作业来执行，并且也可以添加一个新的作业到作业的优先队列中。<br>每日交易时段生成股票报告的应用程序中，需要处理大量数据并且花费很多处理时间。客户向这个应用程序发送请求时，实际上就进入了队列。我们需要首先处理优先客户再处理普通用户。在这种情况下，Java的PriorityQueue(优先队列)会很有帮助。</p>
<p>PriorityQueue类在Java1.5中引入并作为 Java Collections Framework 的一部分。PriorityQueue是基于优先堆的一个无界队列，这个优先队列中的元素可以默认自然排序或者通过提供的Comparator（比较器）在队列实例化的时排序。</p>
<p>优先队列不允许空值，而且不支持non-comparable（不可比较）的对象，比如用户自定义的类。优先队列要求使用Java Comparable和Comparator接口给对象排序，并且在排序时会按照优先级处理其中的元素。</p>
<p>优先队列的头是基于自然排序或者Comparator排序的最小元素。如果有多个对象拥有同样的排序，那么就可能随机地取其中任意一个。当我们获取队列时，返回队列的头对象。</p>
<p>优先队列的大小是不受限制的，但在创建时可以指定初始大小。当我们向优先队列增加元素的时候，队列大小会自动增加。</p>
<p>PriorityQueue是非线程安全的，所以Java提供了PriorityBlockingQueue（实现BlockingQueue接口）用于Java多线程环境。</p>
<p>我们有一个用户类Customer，它没有提供任何类型的排序。当我们用它建立优先队列时，应该为其提供一个比较器对象。</p>
<p><strong>Customer.java</strong></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span>` `com.journaldev.collections;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Customer</span> </span>&#123;</span><br><span class="line">	<span class="keyword">private</span> <span class="keyword">int</span> id;</span><br><span class="line">    <span class="keyword">private</span> String name;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">Customer</span><span class="params">(<span class="keyword">int</span> i, String n)</span></span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.id=i;</span><br><span class="line">        <span class="keyword">this</span>.name=n;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">getId</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> id;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> String <span class="title">getName</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> name;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>我们使用<a target="_blank" rel="noopener" href="http://www.journaldev.com/515/generate-random-number-within-a-range-in-java">Java随机数</a>生成随机用户对象。对于自然排序，我们使用Integer对象，这也是一个<a target="_blank" rel="noopener" href="http://www.journaldev.com/1002/java-wrapper-classes-tutorial-with-examples">封装过的Java对象</a>。</p>
<p>下面是最终的测试代码，展示如何使用PriorityQueue：</p>
<p><strong>PriorityQueueExample.java</strong></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span>` `com.journaldev.collections;</span><br><span class="line"><span class="keyword">import</span> java.util.Comparator;</span><br><span class="line"><span class="keyword">import</span> java.util.PriorityQueue;</span><br><span class="line"><span class="keyword">import</span> java.util.Queue;</span><br><span class="line"><span class="keyword">import</span> java.util.Random;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">PriorityQueueExample</span> </span>&#123;</span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line">        <span class="comment">//优先队列自然排序示例</span></span><br><span class="line">        Queue&lt;Integer&gt; integerPriorityQueue = <span class="keyword">new</span> PriorityQueue&lt;&gt;(<span class="number">7</span>);</span><br><span class="line">        Random rand = <span class="keyword">new</span> Random();</span><br><span class="line">        <span class="keyword">for</span>(<span class="keyword">int</span> i=<span class="number">0</span>;i&lt;<span class="number">7</span>;i++)&#123;</span><br><span class="line">            integerPriorityQueue.add(<span class="keyword">new</span> Integer(rand.nextInt(<span class="number">100</span>)));</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="keyword">for</span>(<span class="keyword">int</span> i=<span class="number">0</span>;i&lt;<span class="number">7</span>;i++)&#123;</span><br><span class="line">            Integer in = integerPriorityQueue.poll();</span><br><span class="line">            System.out.println(<span class="string">&quot;Processing Integer:&quot;</span>+in);</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="comment">//优先队列使用示例</span></span><br><span class="line">        Queue&lt;Customer&gt; customerPriorityQueue = <span class="keyword">new</span> PriorityQueue&lt;&gt;(<span class="number">7</span>, idComparator);</span><br><span class="line">        addDataToQueue(customerPriorityQueue);</span><br><span class="line">        pollDataFromQueue(customerPriorityQueue);</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="comment">//匿名Comparator实现</span></span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">static</span> Comparator&lt;Customer&gt; idComparator = <span class="keyword">new</span> Comparator&lt;Customer&gt;()&#123;</span><br><span class="line">        <span class="meta">@Override</span></span><br><span class="line">        <span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">compare</span><span class="params">(Customer c1, Customer c2)</span> </span>&#123;</span><br><span class="line">            <span class="keyword">return</span> (<span class="keyword">int</span>) (c1.getId() - c2.getId());</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;;</span><br><span class="line">    <span class="comment">//用于往队列增加数据的通用方法</span></span><br><span class="line">    <span class="function"><span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">addDataToQueue</span><span class="params">(Queue&lt;Customer&gt; customerPriorityQueue)</span> </span>&#123;</span><br><span class="line">        Random rand = <span class="keyword">new</span> Random();</span><br><span class="line">        <span class="keyword">for</span>(<span class="keyword">int</span> i=<span class="number">0</span>; i&lt;<span class="number">7</span>; i++)&#123;</span><br><span class="line">            <span class="keyword">int</span> id = rand.nextInt(<span class="number">100</span>);</span><br><span class="line">            customerPriorityQueue.add(<span class="keyword">new</span> Customer(id, <span class="string">&quot;Pankaj &quot;</span>+id));</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="comment">//用于从队列取数据的通用方法</span></span><br><span class="line">    <span class="function"><span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">pollDataFromQueue</span><span class="params">(Queue&lt;Customer&gt; customerPriorityQueue)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">while</span>(<span class="keyword">true</span>)&#123;</span><br><span class="line">            Customer cust = customerPriorityQueue.poll();</span><br><span class="line">            <span class="keyword">if</span>(cust == <span class="keyword">null</span>) <span class="keyword">break</span>;</span><br><span class="line">            System.out.println(<span class="string">&quot;Processing Customer with ID=&quot;</span>+cust.getId());</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>注意我用实现了Comparator接口的<a target="_blank" rel="noopener" href="http://www.journaldev.com/996/java-nested-classes-java-inner-class-static-nested-class-local-inner-class-and-anonymous-inner-class">Java</a><a target="_blank" rel="noopener" href="http://www.journaldev.com/996/java-nested-classes-java-inner-class-static-nested-class-local-inner-class-and-anonymous-inner-class">匿名类</a>，并且实现了基于id的比较器。</p>
<p>当我运行以上测试程序时，我得到以下输出：</p>
<figure class="highlight processing"><figcaption><span>Integer:9</span></figcaption><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br></pre></td><td class="code"><pre><span class="line">Processing Integer:<span class="number">16</span></span><br><span class="line"></span><br><span class="line">Processing Integer:<span class="number">18</span></span><br><span class="line"></span><br><span class="line">Processing Integer:<span class="number">25</span></span><br><span class="line"></span><br><span class="line">Processing Integer:<span class="number">33</span></span><br><span class="line"></span><br><span class="line">Processing Integer:<span class="number">75</span></span><br><span class="line"></span><br><span class="line">Processing Integer:<span class="number">77</span></span><br><span class="line"></span><br><span class="line">Processing Customer with ID=<span class="number">6</span></span><br><span class="line"></span><br><span class="line">Processing Customer with ID=<span class="number">20</span></span><br><span class="line"></span><br><span class="line">Processing Customer with ID=<span class="number">24</span></span><br><span class="line"></span><br><span class="line">Processing Customer with ID=<span class="number">28</span></span><br><span class="line"></span><br><span class="line">Processing Customer with ID=<span class="number">29</span></span><br><span class="line"></span><br><span class="line">Processing Customer with ID=<span class="number">82</span></span><br><span class="line"></span><br><span class="line">Processing Customer with ID=<span class="number">96</span></span><br></pre></td></tr></table></figure>

<p>从输出结果可以清楚的看到，最小的元素在队列的头部因而最先被取出。如果不实现Comparator，在建立customerPriorityQueue时会抛出ClassCastException。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">Exception in thread <span class="string">&quot;main&quot;</span> java.lang.ClassCastException: com.journaldev.collections.Customer cannot be cast to java.lang.Comparable</span><br><span class="line"></span><br><span class="line">    at java.util.PriorityQueue.siftUpComparable(PriorityQueue.java:<span class="number">633</span>)</span><br><span class="line"></span><br><span class="line">    at java.util.PriorityQueue.siftUp(PriorityQueue.java:<span class="number">629</span>)</span><br><span class="line"></span><br><span class="line">    at java.util.PriorityQueue.offer(PriorityQueue.java:<span class="number">329</span>)</span><br><span class="line"></span><br><span class="line">    at java.util.PriorityQueue.add(PriorityQueue.java:<span class="number">306</span>)</span><br><span class="line"></span><br><span class="line">    at com.journaldev.collections.PriorityQueueExample.addDataToQueue(PriorityQueueExample.java:<span class="number">45</span>)</span><br><span class="line"></span><br><span class="line">    at com.journaldev.collections.PriorityQueueExample.main(PriorityQueueExample.java:<span class="number">25</span>)</span><br></pre></td></tr></table></figure>

<h1 id="实现原理："><a href="#实现原理：" class="headerlink" title="实现原理："></a>实现原理：</h1><p>Java中PriorityQueue通过二叉小顶堆实现，可以用一棵完全二叉树表示（任意一个非叶子节点的权值，都不大于其左右子节点的权值），也就意味着可以通过数组来作为<em>PriorityQueue</em>的底层实现。</p>
<p><img src="https://gitee.com/DapeLiu/pic/raw/master/939998-20160512205540484-823563038.png"></p>
<p>上图中我们给每个元素按照层序遍历的方式进行了编号，如果你足够细心，会发现父节点和子节点的编号是有联系的，更确切的说父子节点的编号之间有如下关系：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">leftNo = parentNo*<span class="number">2</span>+<span class="number">1</span></span><br><span class="line"></span><br><span class="line">rightNo = parentNo*<span class="number">2</span>+<span class="number">2</span></span><br><span class="line"></span><br><span class="line">parentNo = (nodeNo-<span class="number">1</span>)/<span class="number">2</span></span><br></pre></td></tr></table></figure>

<p>通过上述三个公式，可以轻易计算出某个节点的父节点以及子节点的下标。这也就是为什么可以直接用数组来存储堆的原因。</p>
<p>PriorityQueue的peek()和element操作是常数时间，add(), offer(), 无参数的remove()以及poll()方法的时间复杂度都是log(N)。</p>
<h1 id="方法剖析"><a href="#方法剖析" class="headerlink" title="方法剖析"></a>方法剖析</h1><h2 id="add-和offer"><a href="#add-和offer" class="headerlink" title="add()和offer()"></a>add()和offer()</h2><p><code>add(E e)</code>和<code>offer(E e)</code>的语义相同，都是向优先队列中插入元素，只是<code>Queue</code>接口规定二者对插入失败时的处理不同，前者在插入失败时抛出异常，后则则会返回<code>false</code>。对于<em>PriorityQueue</em>这两个方法其实没什么差别。</p>
<p><img src="https://gitee.com/DapeLiu/pic/raw/master/939998-20160512205600890-346195840.png"></p>
<p>新加入的元素可能会破坏小顶堆的性质，因此需要进行必要的调整。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line">&#x2F;&#x2F;offer(E e)</span><br><span class="line">public boolean offer(E e) &#123;</span><br><span class="line">    if (e &#x3D;&#x3D; null)&#x2F;&#x2F;不允许放入null元素</span><br><span class="line">        throw new NullPointerException();</span><br><span class="line">    modCount++;</span><br><span class="line">    int i &#x3D; size;</span><br><span class="line">    if (i &gt;&#x3D; queue.length)</span><br><span class="line">        grow(i + 1);&#x2F;&#x2F;自动扩容</span><br><span class="line">    size &#x3D; i + 1;</span><br><span class="line">    if (i &#x3D;&#x3D; 0)&#x2F;&#x2F;队列原来为空，这是插入的第一个元素</span><br><span class="line">        queue[0] &#x3D; e;</span><br><span class="line">    else</span><br><span class="line">        siftUp(i, e);&#x2F;&#x2F;调整</span><br><span class="line">    return true;</span><br></pre></td></tr></table></figure>

<p>上述代码中，扩容函数<code>grow()</code>类似于<code>ArrayList</code>里的<code>grow()</code>函数，就是再申请一个更大的数组，并将原数组的元素复制过去，这里不再赘述。需要注意的是<code>siftUp(int k, E x)</code>方法，该方法用于插入元素<code>x</code>并维持堆的特性。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line">&#x2F;&#x2F;siftUp()</span><br><span class="line">private void siftUp(int k, E x) &#123;</span><br><span class="line">    while (k &gt; 0) &#123;</span><br><span class="line">        int parent &#x3D; (k - 1) &gt;&gt;&gt; 1;&#x2F;&#x2F;parentNo &#x3D; (nodeNo-1)&#x2F;2</span><br><span class="line">        Object e &#x3D; queue[parent];</span><br><span class="line">        if (comparator.compare(x, (E) e) &gt;&#x3D; 0)&#x2F;&#x2F;调用比较器的比较方法</span><br><span class="line">            break;</span><br><span class="line">        queue[k] &#x3D; e;</span><br><span class="line">        k &#x3D; parent;</span><br><span class="line">    &#125;</span><br><span class="line">    queue[k] &#x3D; x;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>新加入的元素x可能会破坏小顶堆的性质，因此需要进行调整。调整的过程为：从k指定的位置开始，将x逐层与当前点的parent进行比较并交换，直到满足x &gt;= queue[parent]为止。注意这里的比较可以是元素的自然顺序，也可以是依靠比较器的顺序。</p>
<p>element()和peek()<br>element()和peek()的语义完全相同，都是获取但不删除队首元素，也就是队列中权值最小的那个元素，二者唯一的区别是当方法失败时前者抛出异常，后者返回null。根据小顶堆的性质，堆顶那个元素就是全局最小的那个；由于堆用数组表示，根据下标关系，0下标处的那个元素既是堆顶元素。所以直接返回数组0下标处的那个元素即可。</p>
<p><img src="https://gitee.com/DapeLiu/pic/raw/master/939998-20160512205615171-1414392177.png"></p>
<p>代码也就非常简洁：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line">&#x2F;&#x2F;peek()</span><br><span class="line">public E peek() &#123;</span><br><span class="line">    if (size &#x3D;&#x3D; 0)</span><br><span class="line">        return null;</span><br><span class="line">    return (E) queue[0];&#x2F;&#x2F;0下标处的那个元素就是最小的那个</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h2 id="remove-和poll"><a href="#remove-和poll" class="headerlink" title="remove()和poll()"></a>remove()和poll()</h2><p><code>remove()</code>和<code>poll()</code>方法的语义也完全相同，都是获取并删除队首元素，区别是当方法失败时前者抛出异常，后者返回<code>null</code>。由于删除操作会改变队列的结构，为维护小顶堆的性质，需要进行必要的调整。</p>
<p><img src="https://gitee.com/DapeLiu/pic/raw/master/939998-20160512205634609-402016454.png"></p>
<p>代码如下：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line">public E poll() &#123;</span><br><span class="line">    if (size &#x3D;&#x3D; 0)</span><br><span class="line">        return null;</span><br><span class="line">    int s &#x3D; --size;</span><br><span class="line">    modCount++;</span><br><span class="line">    E result &#x3D; (E) queue[0];&#x2F;&#x2F;0下标处的那个元素就是最小的那个</span><br><span class="line">    E x &#x3D; (E) queue[s];</span><br><span class="line">    queue[s] &#x3D; null;</span><br><span class="line">    if (s !&#x3D; 0)</span><br><span class="line">        siftDown(0, x);&#x2F;&#x2F;调整</span><br><span class="line">    return result;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>上述代码首先记录0下标处的元素，并用最后一个元素替换0下标位置的元素，之后调用siftDown()方法对堆进行调整，最后返回原来0下标处的那个元素（也就是最小的那个元素）。重点是siftDown(int k, E x)方法，该方法的作用是从k指定的位置开始，将x逐层向下与当前点的左右孩子中较小的那个交换，直到x小于或等于左右孩子中的任何一个为止。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line">&#x2F;&#x2F;siftDown()</span><br><span class="line">private void siftDown(int k, E x) &#123;</span><br><span class="line">    int half &#x3D; size &gt;&gt;&gt; 1;</span><br><span class="line">    while (k &lt; half) &#123;</span><br><span class="line">        &#x2F;&#x2F;首先找到左右孩子中较小的那个，记录到c里，并用child记录其下标</span><br><span class="line">        int child &#x3D; (k &lt;&lt; 1) + 1;&#x2F;&#x2F;leftNo &#x3D; parentNo*2+1</span><br><span class="line">        Object c &#x3D; queue[child];</span><br><span class="line">        int right &#x3D; child + 1;</span><br><span class="line">        if (right &lt; size &amp;&amp;</span><br><span class="line">            comparator.compare((E) c, (E) queue[right]) &gt; 0)</span><br><span class="line">            c &#x3D; queue[child &#x3D; right];</span><br><span class="line">        if (comparator.compare(x, (E) c) &lt;&#x3D; 0)</span><br><span class="line">            break;</span><br><span class="line">        queue[k] &#x3D; c;&#x2F;&#x2F;然后用c取代原来的值</span><br><span class="line">        k &#x3D; child;</span><br><span class="line">    &#125;</span><br><span class="line">    queue[k] &#x3D; x;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>}<br>remove(Object o)<br>remove(Object o)方法用于删除队列中跟o相等的某一个元素（如果有多个相等，只删除一个），该方法不是Queue接口内的方法，而是Collection接口的方法。由于删除操作会改变队列结构，所以要进行调整；又由于删除元素的位置可能是任意的，所以调整过程比其它函数稍加繁琐。具体来说，remove(Object o)可以分为2种情况：1. 删除的是最后一个元素。直接删除即可，不需要调整。2. 删除的不是最后一个元素，从删除点开始以最后一个元素为参照调用一次siftDown()即可。此处不再赘述。</p>
<p><img src="https://gitee.com/DapeLiu/pic/raw/master/939998-20160512205651859-11099237.png"></p>
<p>具体代码如下：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line">&#x2F;&#x2F;remove(Object o)</span><br><span class="line">public boolean remove(Object o) &#123;</span><br><span class="line">    &#x2F;&#x2F;通过遍历数组的方式找到第一个满足o.equals(queue[i])元素的下标</span><br><span class="line">    int i &#x3D; indexOf(o);</span><br><span class="line">    if (i &#x3D;&#x3D; -1)</span><br><span class="line">        return false;</span><br><span class="line">    int s &#x3D; --size;</span><br><span class="line">    if (s &#x3D;&#x3D; i) &#x2F;&#x2F;情况1</span><br><span class="line">        queue[i] &#x3D; null;</span><br><span class="line">    else &#123;</span><br><span class="line">        E moved &#x3D; (E) queue[s];</span><br><span class="line">        queue[s] &#x3D; null;</span><br><span class="line">        siftDown(i, moved);&#x2F;&#x2F;情况2</span><br><span class="line">        ......</span><br><span class="line">    &#125;</span><br><span class="line">    return true;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>


                
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